It is well known in the film industry to have a camera rig consisting of a camera, lens and various other accessories. Typically camera lenses have a focus ring on them that can be turned to adjust the focus of the lens. When the camera rig is in use it can be difficult to access this focus ring and to turn it smoothly, for this reason a device known as a follow focus is often connected to the lens.
A follow focus device typically has a hand wheel that would be positioned on one side of the camera rig, which allows the operator to easily access and rotate the hand wheel. The rotation of the hand wheel is transmitted through the follow focus via a fixed ratio gear train to a final drive roller. The final drive roller will be positioned so that it is in contact with the focus ring of the lens and so will turn the focus ring in proportion to the amount that the user rotates the hand wheel on the follow focus.
The focus ring on the lens typically has gear teeth on it and the final drive roller has matching gear teeth. Because the gear tooth form on the focus rings varies between different lenses it is normal for the final drive roller of the follow focus to be detachable from the follow focus so that different final drive rollers can be used to match the tooth form of the particular lens that is being used. By selecting a final drive roller with the correct tooth form for the lens but greater or fewer teeth it is possible for the user to alter the gear ratio between the hand wheel on the follow focus and the focus ring on the lens. This has the effect of changing the “sensitivity” of the focus ring on the lens to movements of the hand wheel on the follow focus, thus making it easier to either achieve a fast movement from one position to another or giving the user finer control of the position of the lens' focus ring. However changing the final drive gear can only be done when building up the camera rig and cannot then be easily altered between users of the camera rig. Also it means that the operator needs a large number of final drive rollers in order to ensure they have one that will match the tooth form on the focus ring of the lens and give a suitable gear ratio between the hand wheel of the follow focus and the focus ring of the lens.
Because the operator needs extremely good control of the focus ring on the lens it is important that the drive train of the follow focus have zero backlash.
In order to achieve a smooth adjustment of the focus the operator must turn the focus ring at a constant rotational velocity. Due to tolerances and potentially variable mechanical advantage in the drive mechanisms with in the lens itself the torque required to rotate the focus ring varies as it is rotated. This means the operator must match the torque they are applying to the hand wheel to a variable torque requirement in order to maintain a constant velocity. This is a difficult if not impossible task.
Although motor driven solutions to the above outlined problems have been proposed, the current invention seeks to provide a manually operated solution, not reliant on a power supply or motor in order to function. Moreover, the invention can be added to a camera or lens unit as an external add-on focus control device, or retrofit to existing equipment if so desired.